Section 2 – pg 224 Describing Chemical Reactions Chapter 6 Section 2 – pg 224 Describing Chemical Reactions

What are Chemical Equations? Pg 225 What are Chemical Equations? Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction Uses formulas to represent the substances involved in a reaction The formula tells you the ratio of atoms

Structure of an Equation Pg 225 Structure of an Equation A chemical equation tells you the substances you start with in a reaction (reactants) and the substances you get at the end (products)

Conservation of Matter Pg 226 Conservation of Matter Discovered by French chemist Antoine Lavoisier in 1774 Conservation of Matter Principle: during a chemical reaction, matter is neither created nor destroyed The total mass of the reactants must equal the total mass of the products

Pg 226 Conservation of Atoms In chemical reactions, the number of atoms stays the same no matter how they are arranged. So, their total mass stays the same. All the atoms present at the start of the reaction are present at the end of the reaction

Open and Closed Systems Pg 226 - 227 Open and Closed Systems Open System: matter can enter from or escape to the surroundings Any reaction that takes place in a open container Closed System: matter is not allowed to enter or leave Any reaction that takes place in a closed container Closed Open

Balancing Chemical Equations Pg 228 Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation

1. Write the Equation H2 + O2 = H2O Pg 228 1. Write the Equation To begin, write the correct formula for both reactants and products Place reactants on the left side of the arrow, separated by a plus sign, then write the products on the right side H2 + O2 = H2O

Pg 228 2. Count the Atoms Count up the number of atoms of each element on each side of the equation Are they equal? H2 + O2 = H2O 2 Hydrogen ≠ 2 Hydrogen 2 Oxygen 1 Oxygen

3. Use Coefficients to Balance Atoms Pg 228 3. Use Coefficients to Balance Atoms Coefficient: a number placed in front of a chemical formula in an equation You can balance an unbalanced equation by adding coefficients H2 + O2 = H2O 2 Hydrogen ≠ 2 Hydrogen 2 Oxygen 1 Oxygen 2H2 + O2 = 2H2O 4 Hydrogen = 4 Hydrogen 2 Oxygen 2 Oxygen

Pg 229 4. Look Back and Check Check that the total number of atoms stays the same before and after the reaction

Classifying Chemical Reactions Pg 230 Classifying Chemical Reactions The general types of chemical reactions are synthesis, decomposition, and replacement

Pg 230 Synthesis Synthesis: when two or more elements or compounds combine to make a more complex substance Like the combination of hydrogen and oxygen to make water

Pg 230 Decomposition Decomposition: occurs when compounds break down into simpler products

Pg 230 Replacement Replacement: when one element replaces another in a compound, or when two elements in different compounds trade places Things get jumbled up Single Replacement: one element replaces another Double replacement: one element trades places with another

Chapter 6 Section 2 Homework – pg 231

1A. What do the formulas, arrow, and plus signs in a chemical equation tell you?

1B. How are reactants and products treated the same in a chemical reaction? How are they treated differently?

2A. How does the idea of atoms explain the principle of conservation of matter?

2B. If the total mass of the products of a reaction is 250g, what was the total mass of the reactants?

3A. What are three types of chemical reactions?

3B. What is the smallest possible number of products in a decomposition reaction?

3C. Classify the following reaction: P4O10 + 6H2O -> 4H3PO4

Extra Credit: Balance the equations 4. Fe2O3 + C -> Fe + CO2 5 Extra Credit: Balance the equations 4. Fe2O3 + C -> Fe + CO2 5. SO2 + O2 -> SO3